I commented on a similar post to this in the past here Source for TV Show Content Rating - #25 by dokuro
Its a little complicated so please bear with me and I’ll try and explain it as best I understand it.
Plex gets its show level certifications from either TMDB or IMDB depending on your Certification Country settings on your library. However, it will never assign episode certifications from IMDB and will fall back to TMDB for those episodes specific ones.
So if a show has a certification for your country on IMDB and TMDB it will display the cerification in plex for show and episodes. This is because plex scraps the episode certifications from TMDB if they exist there for the show.
However, if a show only has no country certification on TMDB but does on IMDB it will at the show level display the IMDB one but fall back to the US certification for episodes. This is why you’ll see a load of TV-MA episodes certifications. I believe it’ll always fall back to US if nothing else exists.
Best solution is to add accurate certification for your country over at TMDB if you can. However, be aware not all countries are supported on TMDB for shows.
I’m open to correction on the above but this is my personal experience of what’s happening after a good bit of testing.
Edit: I’ve added a UK cerification for the show https://www.themoviedb.org/tv/2428-taggart/changes so assuming the OP has UK set it’ll pickup that change in 24/48 hours.
